1. Field of the Invention
This invention relates to an optical mother disc recording apparatus.
2. Related Background Art
FIG. 1 shows an example of a conventional optical mother disc recording apparatus. Reference numeral 32 denotes an optical mother disc optical mastering disc. The mother disc 32 comprises a glass substrate 33 and a positive-type photoresist layer 34 coated thereon. A turntable 30 carries the mother disc 32 by means of a vacuum adsorption technique (not shown). The turntable 30 is rotated by a spindle motor 31. Thus, a laser beam modulated by information signals moves along the radial direction of the rotating mother disc 32, and irradiates the photoresist layer 34 on the mother disc 32, so that the photoresist layer 34 is exposed.
An optical system of the above apparatus comprises a fixed optical system 10 and a movable optical system 20. The fixed optical system 10 comprises a light source 11 including a laser source such as an Ar gas laser, a reflecting mirror 12, a condenser lens 13, an A/O modulator 14 and an A/O modulator driving circuit 15 for modulating a laser beam in accordance with an information signal, a collimating lens 16, and a reflecting mirror 17. The moving optical system 20 comprises a concave lens 21, a collimating lens 22, a reflecting mirror 23, and an objective lens 24.
A glass plate having a diameter of or more than 200mm and a thickness within the range from 6 to 8 mm is used as a glass substrate of the optical mother disc, because it is easy to grind the glass substrate and to achieve a mechanical strength required for the mother disc. Since the glass substrate has a large size and a large amount of weight, a large size of a spindle motor is required for rotating at a high speed. In order to assure a large enough adsorption area and an adequate support of the mother disc, an outer radius R3 of the turntable has to be larger than an innermost radius R1 (R1&lt;R2) of a recording area A of the optical mother disc 32.
In an area B which has a ring shape having a width of (R3-R1) in the recording area A of the optical mother disc 32, a laser beam which impinges on the photoresist layer 34 enters the glass substrate, and is then reflected by the upper surface of the turntable 30, and the reflection light partially again enters the photoresist layer 34 to expose the photoresist layer 34 twice. Therefore, after the development of the photoresist layer, problems including the decrease in thickness of the film, the transfer of a groove for adsorption arise in the photoresist layer 34 of the area B.
An object of the invention is to overcome the foregoing drawbacks and to provide an optical mother disc recording apparatus which can effectively solve the problems such as the decrease in thickness of film, the transfer of a groove for adsorption, and which has a smaller dimensions.